1. Technical Field
The invention disclosed herein relates to optical barriers and waveguides and the fabrication and use of optical barriers and waveguides for transmitting electromagnetic radiation. Specifically, the invention relates to the fiber-optic sensing under harsh conditions and the fabrication of barriers and waveguides by ion implantation and annealing at high temperatures.
2. Description of Related Art
Sensing and/or monitoring of physical parameters (for example, temperature, pressure, etc.) in harsh environments is critically demanded by many industrial applications, including combustion related applications, such as, in power engines, in power plants, in coal gasification, and in chemical synthesis, among others. The temperatures in such environments typically exceed 1000 degrees C., and in, for example, combustion related applications, high gas pressures and highly reactive chemical species are also involved. The ability for real-time sensing of various physical parameters throughout the combustion systems (for example, in turbine engines) is often essential for gaining an understanding of the system's performance and behaviors, so that further improvements in system performance can be made. In addition, the ability for real-time sensing of the conditions within such processes can provide a means for providing early warning or diagnosis of potential problems in such systems.
Many industrial applications, such as, power engines, power plants, coal gasification and chemical synthesis, among others, are based on combustion processes. To ensure high efficiencies of these applications, it is often necessary to monitor various physical parameters characterizing the combustion processes. Among such parameters, the temporal variation and spatial distribution of temperature over the combustion zones during combustion is particularly important for understanding and controlling combustion processes. The extreme harsh conditions encountered in combustion, typically concerning high temperatures, high gas pressures and high chemical corrosion, have posed great challenges in the sensing and/or monitoring of combustion processes.
Fiber-optics-based sensing technology is particularly attractive for harsh environment sensing. As known in the art, fiber optics sensing is characterized by the transmission of electromagnetic radiation, in particular, light, through a waveguide, for example, a fiber, by the phenomenon of total internal reflection. Compared to other technologies, fiber-optics-based sensors offer many unique advantages, for example, enabling real-time, multi-location monitoring and/or measurement of a wide range of physical and chemical parameters. Unlike silica fibers, which become softened at temperatures at around 800 degrees C., sapphire optical fibers, for example, single-crystal-sapphire optical fibers, can withstand much higher temperatures, for example, exceeding 1000 degrees C. or more. Accordingly, sapphire optical fibers are recognized as sensing material that can be used for optical sensing at temperatures exceeding 1000 degrees C.
However, a major obstacle to the practical implementation of sapphire-based fibers for sensing in harsh environment is the substantial loss of optical signal typically experienced with sapphire fibers. For example, due to the recognized lack of reliable sapphire fiber claddings that are thermally, chemically, and mechanically stable under harsh conditions, the implementation of sapphire-based optical fibers is impractical, except under the most limited environmental conditions. Aspects of the present invention overcome this and other disadvantages of the existing fiber optic sensing.
The challenges associated with harsh-environment sensing can be exemplified in the case of development of next-generation coal gasification technology, as described, for example, by the U.S. Department of Energy (DOE) at http://www.fossil.energy.gov/programs/powersystems/gasification/index.html] and by Higman, et al. (2003). During gasification processes, coal, petroleum, or virtually any carbon-based feed stock in it raw form is broken down into the basic chemical constituents, producing a mixture of carbon monoxide, hydrogen, and other gaseous compounds that can be utilized as fuels or as raw gases for fabrication of valuable chemical products. In a modern gasifier, carbon-containing raw materials are exposed to hot steam and carefully controlled amounts of air or oxygen under high temperatures and pressures. The control of operation temperatures in the gasifier and radiant syngas cooler vessel is often critical for gasification processes. To maximize the efficiency of gasification and to minimize the emission of toxic gases, it is often required to have the real-time temperature distributions within the gasifier. This could be obtained by deploying multiple sensors at various places of the combustion zones and associated components to obtain a real-time 3-dimensional map of the temperature distribution of the system. Typically, the temperatures in the gasifier vary dramatically in a wide range from 500 degrees C. at the syngas cooler to as high as 2000 degrees C. in the combustion zones. These extremely harsh conditions (for example, high temperature, high pressure, and high chemical corrosion) encountered in combustion processes, combined with the desire for real-time monitoring at multiple locations of the combustion system (for example, the length of the combustion zone monitored in the modern gasifiers ranges from 20 to 40 meters [m]) pose a great challenge to existing sensing technologies.
Prior Art Sensing Regimens
As outlined by Nicholas, et al. (2001), conventional methods for temperature detection are based on either the thermoelectric (or Seeback) effect, as in thermocouples, or the thermal radiation effect, as in pyrometers and infrared cameras. As known in the art, the problem associated with thermocouple-based sensing probes is that the sensor performance degrades significantly with time. Moreover, under high-temperature and high-pressure conditions, particularly in the presence of chemically reactive species, the thermoelectric effect of materials can change dramatically due to modification in thermocouple microstructures or/and the formation of insulating layers on thermocouples as a result of slag buildup during combustion processes. These disadvantages of thermocouple-based sensing often result in unreliable temperature sensing or even a complete failure of the thermocouples. In addition, electrical measurements such as thermocouple use should be avoided in situations where the risk of explosion can be potentially high due to electrical sparks.
Regarding the thermal radiation based sensing, despite the advantage of being a non-contact method for temperature measurements, thermal radiation based sensing devices are typically impractical, or even impossible, for large-scale deployment in multi-point sensing applications. Accordingly, the stringent requirements for high temperature sensing under harsh conditions have made the conventional thermoelectric thermal radiation sensing methods inadequate for harsh conditions.
Fiber-Optic Grating Sensors
As a newly emerging technology, fiber-optic sensors have been increasingly used in many industrial applications, for example, as described by Fernando, et al. (2002); Othonos, et al. (1999); Kashyap (1999); Grattan, et al. (1994-2000); Grattan, et al. (2002); Sun, et al. (2000); and Wang, et al. (1992), among others. One important type of fiber-optic sensor is based on the use of “fiber gratings.” As known in the art, a fiber grating structure is a dielectric structure having a periodically alternated refractive index. A fiber grating may typically be fabricated within the fiber core region using ultraviolet or near infrared light illumination combined with techniques such as interferometry or phase masks. Depending on sensing needs, fiber grating sensors can be designed with either “long-period gratings” (LPG) or “short-period gratings,” which are also referred to as “fiber Bragg gratings” (FBGs). In the case of temperature sensing, FBGs are frequently used.
In temperature sensing with FBGs, the short-period modulation of refractive index results in the reflection of a narrow band of the incident optical field within the fiber, with the strongest interaction or mode coupling occurring at the Bragg wavelength (λB), which is defined as a function of temperature (T) by the Equation 1:λB(T)=2nΛ(T)  Equation 1.In Equation 1, Λ(T) is the grating pitch size at temperature T and n is the modal index. In FBGs, the thermal effects on fiber properties can lead to variations in the modal index (n), and/or variations in the grating pitch size (Λ(T)). Therefore, since thermal effects vary the Bragg wavelength, λB(T), the λB(T) is dependent on environmental temperatures and FBG fiber gratings can be used for temperature sensing, for example, in harsh conditions.
In a similar fashion, fiber-grating sensors can be designed for pressure and/or strain detection and chemical sensing, since the index (n) and/or the grating pitch size (Λ(T)) can be varied due to mechanical and chemical interaction between fiber gratings and their sensing surroundings as well.
Compared to conventional sensing methods, fiber-grating-based fiber optics offers unique advantages. For example, fiber-grating-based sensors can be compact, lightweight, and inexpensive to produce. Most importantly, it is easy to multiplex many fiber-grating sensors in a series with a single optical fiber to form distributed sensor arrays for simultaneous sensing at different locations using a single instrument unit. In addition, various types of fiber-grating sensors can be integrated in the same fiber to monitor a variety of physical parameters, including temperatures, pressures, vibration and chemicals. Also, fiber-grating sensors use light as interrogating means and are immune to electromagnetic interference. Moreover, fiber-grating-based sensors can be deployed in places where electrical spark hazards are a concern.
Sapphire-based Sensors
Though silica based fibers can provide effecting sensors, silica-based fibers are not applicable for sensing at ambient temperatures greater than 800 degrees C., because, as noted by Fernando, et al. (2002) and Nubling, et al. (1997), silica-based fibers become softened around 800 degrees C. Since sapphire does not have this limitation, fiber-optics sensors based on sapphire fibers, for example, single-crystal sapphire, have been considered Nubling, et al. (1997) and Pedrazzani (1996) as a promising sensing media for harsh environment sensing.
The thermal properties of single crystal sapphire are very suitable for high temperature sensing. Single crystal sapphire has relatively high thermal conductivity and thermal diffusivity, allowing its temperatures to be rapidly adjusted to the ambient temperatures. The structure of single crystal sapphire is stable up to 1600-1700 degrees C. before it becomes increasingly plastic at temperatures approaching its melting temperature around 2000 degrees C.
Another concern for harsh environment sensing is the chemical reactivity of fiber materials with chemical species present in the sensing environment. Since chemical reactions between fiber materials and their surroundings can substantially change fiber material properties, resulting in severe degradation in sensor performance, and even the complete failure of sensors due to chemical corrosion. For example, silica fibers can be etched off by hydrofluoric acid at room temperature and by other acids like hydrochloric acid and nitric acid at elevated temperatures. However, single crystal sapphire exhibits exceptional chemical inertness and can hardly be attacked by a wide variety of reagents including acids, alkalis, sulfur, and transition metals, even at temperatures greater than 1000 degrees C. In addition, single crystal sapphire has excellent mechanical strength. On the Mohs scale of hardness, which—as known in the art—references the hardness of diamond as 10, the hardness of sapphire is rated at 9. In contrast, the Mohs scale hardness of silica-based glass, from which silica-based fibers are made, is in the range of 4.5-6.5. In addition, sapphire materials also have high resistance to various forms of radiation, including energetic ions, photons (that is, x-rays), and electrons.
Cladding and Sapphire Fibers
Despite its potentials for harsh-environment sensing applications, sapphire-fiber technology is in its infancy compared to the much more mature silica-fiber technology. Table 1 identifies and compares the characteristics of silica and sapphire fibers. Though, as shown in Table 1, sapphire fibers have marked differences in characteristics that suggest sapphire is preferable to silica for harsh environments, the major issue limiting the use of sapphire-based fiber sensors in an industrial setting is the lack of reliable claddings for sapphire fibers, particularly at temperatures above 1000 degrees C.
TABLE 1Comparison of Silica and Sapphire Fiber Characteristics(Source: Fernando, et al. (2003))Max.RefractiveYoung'sOpticalReliableMin.fiberMax. sensingindexmodulusattenuationcladdingfiber sizelengthtemperatureSilica1.46414 GPa0.2 dB/mYes <10 μm>10 km <800° C.Sapphire1.76 73 GPa  1 dB/mNo>100 μm <3 m>1500° C.
As known in the art, fiber “cladding” is the layer of material of lower refractive index that is typically in intimate contact with a core material of higher refractive index, for example, silica or sapphire. The presence of the cladding causes light to be confined to the core of the fiber by total internal reflection at the boundary between the core and the cladding. (“THE BASICS OF FIBER OPTIC CABLE,” http://www.data-connect.com/Fiber_Tutorial.htm, included by reference herein.)
In addition, small size sapphire fibers (that is, having an outside diameter less than 10 micrometers [μm]) are currently unavailable in the field, and thus single-mode optical propagation within sapphire fibers is essentially impossible, which is another drawback for sapphire-based fiber-optics sensing.
As known in the art, the refractive index of fiber claddings should be less than that of the fiber core where light is confined to propagate. Although the refractive index of air is less than that of fiber materials, air cannot serve as an effective cladding for sapphire fibers. Due to the large difference in refractive index between sapphire (˜1.76) and air (˜1.0), light propagation in sapphire fibers is characterized by very large numerical apertures, high degree of multimode, and high sensitivity to bending. These characteristics of sapphire fibers can cause difficulties for the fabrication and operation of sapphire fiber sensors. Moreover, a further disadvantage of sapphire-fiber-based sensing is that the intensity of optical signals in sapphire fibers can be significantly decreased as a result of strong light scattering at the fiber-air interface, if the fibers are not properly clad. This is particularly severe for sensing applications in harsh environments where constantly varied surrounding atmospheres, deposited dust, and defects (for example, scratches and/or cracks) can develop on the fiber surface, among other places, that can result in unsatisfactory optical signal losses. Accordingly, the use of sapphire-based optical sensors can be impractical, or even substantially impossible, for delivering and detecting optical signals when a large fiber length (that is, greater than 0.5 m) is needed, as in the case of multi-point sensing for oversized facilities, such as, power plants and gasifiers.
In addition, for fiber-optics sensing applications, often the number of optical propagating modes must be minimized. However, the presence of cladding can also affect the number of propagating modes allowable within fibers. For sapphire fibers, the number of allowable propagating modes can be decreased by over 80% when the fibers are properly clad by materials with the refractive index a few percent less than that of sapphire.
Unlike silica-based fibers, core-clad structures for single crystal sapphire fibers are unattainable during their growth. Accordingly, one commonly researched method for sapphire fiber cladding is to coat the fiber surface with a layer of dissimilar material with lower refractive index, as described by Desu, et al. (1990); Raheem-Kizchery, et al. (1989); and Davis, et al. (1993). Many materials, including polycrystalline alumina, metal niobium, silicon carbide and silicon oxynitride, have been tried as coating materials for sapphire. Another cladding method proposed in U.S. Pat. No. 6,968,114 of Janney, et al. is based on chemical reactions between a compound powder (for example, MgO) and the sapphire to convert the surface sapphire into a layer of spinels (for example, MgAl2O4) that could help light propagate in the underlying sapphire fiber core. However, none of these efforts has yielded satisfactory performance for sapphire fiber cladding in harsh environments, particularly at temperatures around and above 1000 degrees C.
In addition to the requirements of possessing a lower refractive index (that is, compared to single-crystal sapphire fibers) and being transparent over the operating optical spectral range, the desired cladding material should match sapphire closely in thermal expansion and conduction properties. For ambient temperatures above 1000 degrees C., even a small difference in such thermal characteristics between sapphire and the cladding material can result in mechanical failures (for example, cracking and/or delamination of the cladding from the fiber surface) of claddings due to stress buildup.
Another important aspect is the chemical stability of cladding materials under high temperature, high pressure, and high chemical corrosion conditions. On one hand it is desired to form strong bonding between sapphire fibers and cladding structures in order to enhance adhesion between them, and on the other hand the cladding layers should remain inert to those active chemical species present in sensing environments, otherwise chemical erosion of claddings would occur during sensing periods.
It is well known in the art that, with the present state of the art, it is very difficult (if not impossible) to obtain a cladding material that satisfies all these stringent requirements that can be used effectively for sapphire fiber cladding, for example, for harsh environment sensing applications. Consequently, the inventors have found that there has been no viable technology available in the art that would allows sapphire fibers to be properly clad for use under harsh-environment conditions, for example, involving high temperatures (for example, greater than 1000 degrees C.) and resistance to high chemical corrosion and/or erosion.
Modification of Refractive Index
As is known in the art, the dielectric constant of a material has three components, that is, 1) the electronic polarization, 2) the distortion polarization, and 3) the orientation polarization. The displacement of bound electrons or constituent ions in the presence of an external electric field is responsible for the electronic polarization or the distortion polarization, respectively. The orientation polarization results from electric field induced motion of the molecular units.
At the optical frequency and beyond, only the electronic polarization contributes to the dielectric constant since the other two types of motion cannot vary in time with the electric field. At such high frequency, the refractive index (n) can be related to the dielectric constant (∈) in a simple way, that is, by Equation 2,∈=n2  Equation 2As known in the art, for example, as described by Maex, et al. (2003), Equation 2 yields the “Lorenz-Lorentz” (L-L) equation, that is, Equation 3.
                                                        n              2                        -            1                                              n              2                        +            2                          =                                            4              ⁢              π                        3                    ⁢          ρα                                    Equation        ⁢                                  ⁢        3            In Equation 3, ρ is the “atomic concentration” and α is the “molecular electronic polarizability.” The Lorenz-Lorentz equation (that is, Equation 3) suggests that the refractive index (n) of a material can be reduced by decreasing the atomic concentration (ρ) and/or by decreasing the molecular electronic polarizability (α). As known in the art, the electronic polarizability (α) depends on the chemical bonding of the material, and a dramatic reduction in the electronic polarizability is typically difficult to achieve without significant change of constituting chemical species in the material matrix. However, the effect of atomic concentration (ρ) on the refractive index (n) can be relatively easier to realize, since it is possible to vary a material's atomic concentration by modifying its structural properties. For example, the introduction of pores or voids in the material can lower the local density at the pore or void locations and the atomic concentration (ρ) and, therefore, the refractive index (n) at these locations.Ion Beam Modification of Refractive Index
Ion implantation has been established as a very powerful method for modifying the optical properties in materials, for example, as reported by Townsend, et al. (1994). During ion implantation, energetic ions (that is, ions having a typical energy spanning from a few hundreds of kilo-electron-volts [keV] to tens of million electron-volts [MeV]) are typically impinging on a material. During this impingement, various ion-solid interactions take place as the energetic ions traverse the material, and typically lead to modifications in material properties.
As understood in the art, in the early stage of ion impingement, that is, when the energetic ions pass across the surface region of the material, interactions between incident ions and target electrons contribute overwhelmingly to the energy loss of the ions. As a result of energy transfer to target electrons, atomic excitation and ionization occur in the near surface region, causing changes in a material's bonding structures (for example, rearranging or breaking of chemical bonds). Near the end of the projected ion range in the material, the ion energy is significantly reduced to about tens of keV. At such low energy regimes, it is understood that incident ions lose their energy mainly by colliding with the target nuclei. These nuclear collisions displace the constituent atoms from their original locations, leaving a high density of defects (for example, voids, vacancies, and/or interstitials) in the material. As the incident ions finally settle down in the material, additional changes in the electronic structure and defect formation, for example, due to chemical bonding between implanted impurities and their surroundings, can be induced in the material depending on the implanted ion species and the target material.
The resulting modifications in material properties, including local atomic concentration, defect formation, chemical bonding arrangements, and electronic environments, can have a profound effect upon the optical properties of ion implanted materials. For example, it is well documented in the art that ion implantation can be used for tailoring the profile of refractive index in many materials to fabricate planar optical waveguides, for example, see Townsend, et al. (1994); Laversenne, et al. (2004); Kostritskii, et al. (2007); Wang, et al. (2007); Szachowicz, et al. (2007); and Chen, et al. (2005).
Compared to other techniques, the ion implantation method has many unique advantages. For example, the location and the width of the optical confinement region can be precisely controlled by varying the ion implantation conditions. In addition, ion implantation can allow optical waveguides to be made deep below the surface, ensuring a good mechanical, chemical and thermal reliability of the device. This is of particular importance for applications under harsh conditions.
Ion implantation can produce both positive and negative variations in refractive index. Typically, implantation of heavy ions (for example, silicon [Si]) results in increased index of refraction in the implanted regions, as noted by Townsend, et al. (2004) and by Hu, et al. (2001), while light ions (for example, hydrogen [H] or helium [He]) implantation tends to decrease the refractive index in the implanted region, as reported by Townsend, et al. (1994) and Laversenne, et al. (2004). Compared to the case for heavy ions, the defect production rate for light ions in materials is much lower and therefore the use of light ion implantation is very attractive for fabrication of buried waveguides in materials.
The particular structural modifications induced by light ion implantation in crystals, as reported by Terreault (2007), are responsible for the reduction of refractive index in the implanted regions. Ion species like H or He are insoluble in crystals, and when incorporated in crystals, they tend to accumulate preferentially at sites of defects (typically at vacancy-type defects), leading to the formation of molecular (for example, H2) or monoatomic (for example, He) gas phases in the crystals. The high pressure buildup associated with the resultant gas states in the crystal matric can result in a volume expansion or a decrease in the local atomic density and a decrease in the local atomic concentration (ρ), and, correspondingly, a decrease in the refractive index (ρ) according to the Lorenz-Lorentz equation shown in Equation 3. The formation of gas phases is believed to be significantly accelerated during post-implantation thermal annealing. However, above certain temperatures, the formed gases can be completely driven out, leaving behind a band of voids or cavities of nanometer [nm] scale size (for example, 1-100 nm) in the materials. The density and the size of these voids are dependent on annealing conditions (for example, temperature and time). As expected from the Lorenz-Lorentz equation (Equation 3), the refractive index (n) in the region containing the voids and/or cavities would be further decreased due to lower local atomic density. Formation of submicron-sized void/cavity in crystals due to hydrogen or helium ion implantation has been reported for various materials from elemental semiconductor crystals like Si, to compound ionic crystals like GaN and ZnO, see Terreault, et al. (2007); Tong, et al. (1997); Hong, et al. (2007); Kucheyev, et al. (2002); and Singh, et al. (2007). Once formed in materials, such cavities/voids can even survive the harsh conditions (for example, high temperatures, pressures, etc.) encountered in nuclear reactors. For example, a notorious example is the embrittlement of reactor materials due to formation of voids following prolonged exposure to energetic protons or helium ions released from nuclear reactions, as described by Jung, et al. (2001).
There have been several investigations on the use of H or He ion implantation for fabrication of buried planar waveguides in optical materials, including LiNbO3 [Laversenne, et al. (2004)], Y3Al5O12 [Szachowicz, et al. (2007)] and Al2O3 (sapphire) [Townsend, et al. (1994) and Grivas, et al. (2006)] crystals. A common feature for these prior art waveguides is that light is confined deep below the surface by an optical barrier produced by H or He ion implantation. As an example, Laversenne, et al. (2004) recently demonstrated, the use of 0.4-1.5 MeV protons to achieve buried planar waveguides with good performance in sapphire crystals.
However, the present inventors have found that this prior art has been limited to room temperature or relatively low processing temperatures conditions for the fabrication and operation of ion implantation produced waveguides, specifically to processing temperatures less than 200 degrees C. Accordingly, the prior art teachings, waveguides, and barriers are not applicable to conditions for higher temperatures, for example, temperatures greater than 200 degrees C., that is, those temperatures that characterize the harsh environments typically encountered in certain industrial processes, as discussed above. Aspects of the present invention overcome this and other disadvantages of the prior art that, among other things, provide waveguides and barriers that can be used effectively in harsh environments, that is, where the prior art waveguides cannot.